The Drosophila body plan Flashcards
Where is segmentation present in the drosophila?
All along the body
In the head
In the larvae
Where are the legs present in drosophila?
One pair of legs in each of the 3 thoracic segments
What did screening of drosophila mutations by Nusslein-Volhard and Wishaus show?
Many different phenotypes, all characterised into 4 groups
What are the 4 groups that the mutations were classified into?
1) Maternal genes
2) Gap genes
3) Pair rule genes
4) Segment polarity genes
What happens when mutate gap genes?
There is a gap in the body plan - loss of segments in the middle
What happens when mutate paired-rule genes?
Miss EVERY OTHER segment in the body
What are examples of genes that fall into the different categories of mutations?
1) Maternal - Bicoid
2) Gap - Knirps
3) Paired rule - even-paired, odd-paired
4) Segment polarity - Gooseberry
What happens if mutate segment polarity gene?
Loss of the naked cuticle between the denticle belts - looks fuzzy
How are genes named?
After their phenotype when they are mutated
What is the hierarchy of genes in the drosophila?
- Genes that progressively define and divide up the egg
- Where expression of the genes at ONE LEVEL is needed for the expression of the genes at the NEXT LEVEL
- Maternal
- Gap
- Paired-rule
- Segment polarity
What type of molecule in bicoid?
A DNA binding transcriptional ACTIVATOR
Where is bicoid made?
It is loaded into the developing oocyte from the nurse cells
What protein is made from bicoid mRNA?
Bicoid protein - a MORPHOGEN
What happens to the bicoid protein when it is translated from bicoid mRNA?
It diffuses away from the anterior of the egg to form a GRADIENT
When are the high affinity binding sites for the bicoid morphogen activated?
At LOW concentrations of the morphogen
When are the low affinity binding sites for the bicoid morphogen activated?
At HIGH concentrations of the morphogen
What happens if there is a mutation in bicoid?
There is a loss of ANTERIOR structures
What happens if inject anterior CYTOPLASM into an anterior mutant?
What does this show?
There is PARTIAL RESCUE (development of SOME anterior structures)
Shows that there is a diffusible molecule doing this (as it is present in the cytoplasm)
What happens when the donor anterior cytoplasm is injected into the middle of the bicoid mutant?
Head-like structures in the middle and thoratic segments moving outwards (mirror image)
How are paired-rule genes expressed with no bicoid mutant?
7 bands, with a small gap at the front and the back
What happens to the expression of the pair-rule genes in a bicoid mutant?
Band spread out, some travel anteriorly (smaller gap between the anterior and the first band of expression)
What happens to the expression of the pair-rule gene when bicoid is overexpressed?
7 bands are pushed towards the POSTERIOR (larger gap between the anterior and the first band)
What are ALL gap genes a readout of?
How in the case of Krupple?
What is the pattern of expression with Krupple?
The anterior-posteiror BICOID GRADIENT
Above certain threshold = off
Below certain thereshold = off
Between these 2 thresholds (Goldilocks zone) = on
Krupple - One stripe of expression in the centre
How are different Gap genes expressed?
Different stripes of expression that don’t always align
Different Gap genes control different parts of the drosophila embryo
Which gap genes are transcriptional repressors?
Giant and Krupple
What turns off the expression of the pair-rule gene eve (even-skipped)?
So what do these proteins define?
Giant and Krupple
Define the boundaries of eve expression
What turns on the expression of the pair-rule gene eve (even-skipped)?
Bicoid and Hunchback (another gap gene)
Where is the gap gene Hunchback expressed?
At the anterior HALF
How are each of the 7 stripes of pair-rule gene expression specified?
Independently of each other
By a different combination of positive and negative acting transcriptional regulators
How many odd and even segments are there?
7 of each = 14
At what point are the segments defined?
After the expression of pair-rule genes
How do parasegments relate with segments?
Parasegments are defined before segments
Parasegments are the gene expression boundaries but segments are the physical indentations that form in the embryo
Parasegments are half a segment out of phase with segments
So, when segmentation occurs, the anterior of the parasegment becomes the posterior of the segment
So each segment is comprised of the posterior part of one parasegment and the anterior of the next parasegment
What does each segment of the drosophila contain?
Denticle belt and the naked cuticle
What are examples of segment polarity genes?
Hh (hegehog)
Wg (wingless)
Where are Hh and Wg expressed?
On NEIGHBOURING cells
What is the expression of Wg dependant on?
What is the expression of Hh dependant on?
What does this cause?
They are both dependant on each other
Causes a mutual feedback loop:
- Feedback onto each other to maintain the expression of each other
- Refine parasegment borders (within segments)
What happens if there is a loss of Hh signalling?
Loss of Wnt signalling?
Loss of Hh –> loss of Wnt
Loss of Wnt –> loss of Hh
What suppresses denticle development and maintains naked cuticle?
What does this maintain?
Wg expression, which is maintained by Hh expression
What gradient is formed by Wnt and why?
Asymmetric morphogen gradient that is stretched out more to the anterior (weaker anteriorly)
Due to:
- HSPGs
and
- The expression of receptors
What are the ‘selector genes’?
What do they encode?
Hox (homeotic) genes
Encodes segment identity
How are hox genes expressed along the AP axis?
In the SAME order as the genes are within the genome
What is the expression of the Hox genes controlled by?
A combination of gap and pair-rule genes
What are hox genes and where do they bind?
Transcription factors
All bind to the SAME DNA sequence but cause a DIFFERENT output
What happens when there is a mutation in antennapedia?
The antennapedia change to legs
What is a ‘long germ band’ insect?
Drosophila
All 14 segments are defined at ONCE
How does segmentation occur in a ‘short germ band’ insect?
- Start with head and thoracic segments
- Add ABDOMINAL segments SEQUENTIALLY
In short germ band insects, what is the addition of abdominal segments at the expense of?
- The proctodeum (the posterior disc) which becomes smaller and smaller
- Appears to ‘bud off’ segments
What is the short germ band mechanism of segmentation likely to represent?
Why?
The ancestral segmentation mechanism
As short band insects were around before the long band insects
Which of the short germ band or long germ band mechanism is easier?
Why?
Short germ band:
- Only requires system LOOPING (adding one segment after the other
- Whereas the long germ band involves many genes (maternal, gap, pair rule) which are different in each segment
What can be seen in strigamia maritima?
- ‘Growth rings’ in Delta and Notch
- System of feedback loops where notch causes the expression of Her gene
Describe the feedback loop in strigamia maritima
- Notch causes the expression of her
- Her is a negative regulator that switches off delta expression (normally high)
- So when notch increase, delta decrease
- Decrease in delta - decrease in notch in a neighbouring cell (Delta increase in this cell)
- Forms a CYCLE
What is the segmentation clock?
Describe this clock
The delay in the activation of notch and the decrease in delta in strigamia maritima:
- Time lag causes oscillations between strong and weak signalling
- Propagation of the signal causes a wave of activation from the posterior
What mechanism do short germ band insects use to segment?
Segmentation clock
What does the flower beetle use to segment?
A mixture of both systems
